From the international patent application WO87/05005 made public on Aug. 27, 1987 a method and an apparatus for the purpose of treating sludge is known. The dewatering process described therein is discontinuous and to obtain a satisfying capacity the apparatus has to be given rather large dimensions. However, the construction of the dewatering apparatus is often restricted by the size of the basin or container in which the dewatering apparatus is to be inserted.
The reason to the discontinuous working of the apparatus described in WO87/05005 is that the screened jacket walls during the dewatering process gradually are obstructed by sludge and the removal of reject water through the walls will be more and more slow and finally being completely stopped. The working time up to such a complete obstruction of the dewatering process will depend on the quantity of solids in the sludge to be dewatered. Thus, the dewatering time is shorter the thicker the sludge is. Also other factors are affecting the time of the dewatering process, such as e.g. the use of an unsuitable polymer, insufficient mixture of the polymer in the sludge, the quantity of fat in the sludge, etc. The apparatus known from WO87/05005 requires cleaning intervals during which the screened jacket walls are cleaned by flushing means.
To be able to use flushing means in the dewatering apparatus known per se the basin or the container within which the apparatus is introduced has to be emptied of sludge. Thus, the effective working time for the dewatering process is decreased and has to be interrupted by cleaning periods when the basin or container is emptied.